The aim of this project is to determine the source of bone cells in the normal endosteal bone remodeling sequence. Considerable evidence has accumulated suggesting that the precursor cell of the osteoclast comes from the circulation. The precursor cells therefore, should be transferrable from one animal to another under appropriate conditions. It is also known that the injection of spleen or bone marrow cells from a F-1 hybrid (donor) into an irradiated parent (recipient) will not lead to a host versus donor rejection reaction, but will allow the establishment of a line of cells in the recipient with surface markers different from those of the original recipient cells. By utilizing available inbred strains of mice, their progeny and appropriate antisera against major H-2 surface histocompatibility antigens, it should be possible by immunofluorescent markers to detect and identify donor cells. By these methods, the question of the origin of the osteoclast from circulating precursor cells may be resolved.